1. Field of the Invention
This invention relates to a developer carrying member used when an electrostatic latent image formed on an electrostatic latent image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member is developed. It also relates to a developing apparatus, a developing method, an image forming apparatus and a process cartridge using such a developer carrying member.
2. Related Background Art
As a developing apparatus used when electrostatic latent images formed on a photosensitive drum serving as an electrostatic latent image bearing member are developed by the use of a magnetic toner as a one-component type developer, a developing assembly as shown in FIG. 7 is known in the art. As shown in FIG. 7, a developer container 53 as shown in FIG. 6 holds a magnetic toner 54 as one-component type developer, and electric charge having a polarity reverse to the electric charge of the electrostatic image formed on a photosensitive drum 51 and to the development standard potential is imparted to magnetic toner particles by the friction between particles of the magnetic toner and the friction between a developing sleeve 58 as a developer carrying member and the magnetic toner particles. The magnetic toner thus charged is applied on the developing sleeve 58 by means of a magnetic blade 52 and then transported to the developing zone D at which the photosensitive drum 51 and the developing sleeve 58 face each other, where the magnetic toner carried on the developing sleeve 58 by the action of the magnetic field formed by a magnet 55 stationarily set therein is attracted to develop the electrostatic latent image on the photosensitive drum 51. Letter symbols A and B denote the rotating directions of the developing sleeve 58 and the photosensitive drum 51, respectively. Reference numeral 59 denotes a development bias means for applying a development bias voltage at the time of development; and 60, an agitating blade for agitating the magnetic toner 54 inside the developer container 53.
When, however, such a one-component type developer is used, it is difficult to control the toner charging. Although various trials have been applied to developers, the problems concerning non-uniformity of charging and running instability of charging are not completely solved.
Especially as the developing sleeve is repeatedly rotated, the charge quantity of the toner applied on the developing sleeve becomes too large by contact with the developing sleeve, so that the toner and the developing sleeve attract each other on account of the reflective force of the developing sleeve surface and the toner turns immobile on the surface of the developing sleeve, thus, it does not move from the developing sleeve to the electrostatic latent image bearing member (drum). Such a phenomenon, what is called "charge-up", tends to occur. Once such charge-up has occurred, the toner forming an upper layer on the sleeve is hard to electrify and the quantity of toner participating in development is reduced, bringing about problems of, for example, line images being thinner and image density of solid images being thinner.
Moreover, the toner layer may be formed in a different state at image areas (areas where toner is consumed) and non-image areas to have been charged in different conditions. Hence, when, e.g., a position where a solid image with a high image density has been once formed by development comes to the development position according to the next rotation of the developing sleeve and a latent halftone image is developed, a sign of the solid image may appear on the developed halftone image. Such a phenomenon, what is called "sleeve ghost", tends to occur.
Recently, in order to make electrophotographic image quality much higher, toners have been made smaller in particle diameter and made finer. For example, in order to improve image quality such as resolution and sharpness to faithfully reproduce electrostatic latent images, it is common to use toners with a weight average particle diameter of about 6 to 9 .mu.m. Also, for the purpose of making copying time shorter and power consumption smaller, there is a tendency toward lower fixing temperature. Under such circumstances, the toner more tends to electrostatically adhere onto the developing sleeve and at the same time undergo external physical force, so that contamination of the developing sleeve surface and toner fusing are liable to occur.
As a method for preventing such phenomenons, it is proposed to use in a developing apparatus a developing sleeve comprising a metal substrate provided thereon with a coat layer formed of a resin in which a solid lubricant and a conductive fine powder such as carbon powder are dispersed. According to the use of this method, the above phenomenons seem to greatly decrease. In this method, however, the surface of the developing sleeve is not sufficiently even in its shape and also the surface of the developing sleeve has a smaller area to which triboelectric charges are imparted, so that uniform charging of toner and a rise in toner charging (or the quick electrification of toner) can not be sufficient in some cases. Accordingly, black spots around character line images may occur and image density may lower in an environment of high temperature and high humidity. Thus, this method is still not well satisfactory, which also leaves a problem concerning running performance because the coat layer may become brittle.
Japanese Patent Application Laid-open No. 3-200986 discloses a method in which a developing sleeve comprising a metal substrate provided thereon with a conductive coat layer formed of a resin in which a solid lubricant, a conductive fine powder such as carbon powder and also spherical particles are dispersed is used in the developing apparatus. When this method is used, the developing sleeve surface can have an even shape, charging can be uniform and wear resistance can be improved. However, even in this developing sleeve, its running performance is sought to be more improved, e.g., to be improved in the ability to impart quick and uniform charging to toner and in wear resistance of the conductive coat layer, and to prevent toner contamination and melt-adhesion of toner once the sleeve has worn.
Japanese Patent Application Laid-open No. 2-176762 discloses a method in which a developing sleeve containing a charge control agent in a coat layer formed on the surface of the developing sleeve is used in the developing apparatus. When this method is used, a rise in toner charging and the uniform charging of toner can be improved to a certain extent, but the surface of the developing sleeve can not still have a charge-providing ability good enough to be well effective for a high image quality superior in character line sharpness and for the stability of image density in an environment of high temperature and high humidity. Also, this method can not still be satisfactory in respect of running performance, and is sought to be further improved.